1. Field of Invention
The present invention relates to an optical system and optical apparatus. More particularly, the present invention relates to an optical system and optical apparatus capable of switching between OCT (Optical Coherence Tomography) observation and OCM (Optical Coherence Microscopy) observation.
2. Discussion of Related Art
A scanning optical microscope having a confocal optical system disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. Sho 61-219919 is known as an optical apparatus that allows observation of the inside of a biological sample. Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 4-146410 states techniques relating to a scanning optical system whereby the beam diameter of light incident on an objective is changed and the position of a scanning mirror is adjusted in accordance with the size and position of the pupil of the objective.
Recently, a technique called “low-coherence interferometry” or OCT (Optical Coherence Tomography)” such as that disclosed in U.S. Pat. No. 5,321,501 has become known as a method that allows observation of the inside of an opaque scattering sample, e.g. a biological tissue. FIG. 11 shows a general optical system for the low-coherence interferometry. Light from a light source 81 with a short coherence length is split by a beam splitter 82 between a signal light path leading to a sample 4 and a reference light path leading to a reflecting mirror 83. Light going and returning along the signal light path and the reference light path are recombined in the beam splitter 82. At this time, because the signal light path forms an optical path length substantially equal to that of the reference light path at an observation point 86 in the sample 4, only light scattered back from a region at the observation point 86 within a range in the optical axis direction that is substantially equal to the coherence length interferes with the reference light. Accordingly, by detecting the resulting interference signal with a detector 84, information about the inside of the sample 4 can be selectively obtained in the optical axis direction.
In general, the reflecting mirror 83 in the reference light path is moved in the optical axis direction, thereby performing scanning in the direction of depth of the sample 4 and, at the same time, producing a Doppler shift in the reference light. With the low-coherence interferometry, in general, heterodyne interferometric measurement is carried out to detect a beat signal having a Doppler shift frequency in the interference signal. Therefore, the measurement can be performed with a very high S/N ratio.
By performing scanning also in a direction perpendicular to the optical axis with a scanning mirror, an image of the xz-section in FIG. 11 can be obtained. If the depth of focus of the objective is set greater than the movable range of the reflecting mirror in the reference light path, the resolution in the xy-plane can be kept substantially constant despite the movement of the reflecting mirror. If the reflecting mirror is moved at high speed, image acquisition can be performed at high speed. In this case, an objective having a small numerical aperture is used because a large depth of focus is needed.
Meanwhile, a microscopic observation method using a confocal optical system with an objective having a large numerical aperture is known as a low-coherence interferometric technique, as shown in “Optics Letters, Vol. 19, No. 8, p. 590 (1994). The observation method is known as “OCM (Optical Coherence Microscopy)”, which is a microscopic technique in which the high spatial resolution of the confocal optical system and the high S/N ratio of the low-coherence interferometry are combined together. In OCT, the coherence length is short in comparison to the depth of focus of the objective, whereas in OCM the depth of focus of the objective is equal to or less than the coherence length. OCM makes good use of the merit that the resolution in the xy-plane in FIG. 11 is high, and uses scanning mirrors for both the x- and y-directions to perform scanning in directions perpendicular to the optical axis, thereby making it possible to obtain a high-resolution image of the xy-plane. OCM also allows the inside of a living body to be observed at high resolution by making use of heterodyne interferometric measurement as in the case of OCT. The technical features of OCT and OCM are also stated in “Optics & Photonics News” May, p. 41 (1997).
As has been described above, OCT allows observation over a wide range in the direction of depth of the sample (in the z-direction). However, OCT is incapable of observing the sample with high resolution at a position of certain depth. Conversely, OCM allows the sample to be observed with high resolution at a position of certain depth but suffers from the problem that it takes a great deal of time to find a desired depth position because the depth of focus is shallow.